1. Field of the Invention
The present invention relates to secondary batteries, and, more particularly, to a modified cap assembly plate for a lithium secondary battery providing improved reliability and safety characteristics.
2. Description of the Prior Art
Recently, electronic appliances having compact sizes and light weight, such as cellular phones, notebook computers and camcorders, have been actively developed and produced. Such electronic appliances are equipped with battery packs so that users can use the electronic appliances in various places even if electric power sources are not separately provided for the electronic appliances. The battery pack includes at least one bare cell capable of outputting an operational voltage having a predetermined level in order to operate the electronic appliance for a predetermined period of time.
Secondary batteries, which are rechargeable batteries, are currently employed in the secondary pack due to their economical advantages. Secondary batteries include Ni—Cd batteries, Ni-MH batteries and Li secondary batteries, such as Li batteries or Li-ion batteries.
In particular, lithium secondary batteries have an operational voltage of about 3.6V, which is three times higher than that of Ni—Cd batteries or Ni-MH batteries used as power sources for the portable electronic appliances. In addition, lithium ion secondary batteries have high energy density per unit weight so they are extensively used in the advanced electronic technology fields.
Lithium secondary batteries typically use lithium-based oxide as a positive electrode active material and carbon as a negative electrode active material. In general, lithium batteries are classified into liquid electrolyte batteries or polymer electrolyte batteries according to the kind of the electrolytes used therefor. Liquid electrolyte batteries are called “lithium ion batteries” and polymer electrolyte batteries are called “lithium polymer batteries.” In addition, lithium secondary batteries may be fabricated with various shapes, such as cylinder type lithium secondary batteries, square type lithium secondary batteries, or pouch type lithium secondary batteries.
Typically, lithium ion secondary batteries include an electrode assembly having a positive electrode plate coated with a positive electrode active material, a negative electrode plate coated with a negative electrode active material and a separator interposed between the positive electrode plate and the negative electrode plate for preventing a short circuit while allowing the movement of lithium-ions exclusively. Lithium ion secondary batteries also typically include a case for receiving the electrode assembly and an electrolyte contained in the case in order to enable lithium-ions to move.
In such a lithium ion secondary battery, the positive electrode plate coated with the positive electrode active material is connected to a positive electrode tap and the negative electrode plate coated with the negative electrode active material is connected to a negative electrode tap. The positive electrode plate and the negative electrode plate are stacked with the separator therebetween and wound in the form of a jelly roll, thereby forming the electrode assembly. The positive electrode active material is composite oxide including lithium (Li). Typically, the positive electrode active material includes LiCoO2, which is obtained by mixing lithium carbonate and cobalt oxide in a ratio of about 1.2:1 and performing a plastic process under the temperature of between about 400 to 1000° C.
The electrode assembly is then accommodated in the case and a cap assembly is sealed to an upper end of the case in order to prevent the electrode assembly from separating from the case.
After sealing the cap assembly to the upper end of the case, the electrolyte is injected into the case and an electrolyte injection hole formed in the case is sealed, thereby providing the lithium ion secondary battery.
As shown in FIGS. 1A and 1B, the cap assembly includes a conventional cap plate 100 formed with a safety vent 110 for preventing accidental explosion of the lithium ion secondary battery caused by high internal pressure of the battery from gas generated during the charge operation of the lithium ion secondary battery. When the internal pressure of the lithium ion secondary battery reaches an unsafe level, the safety vent 110 fractures to allow gas contained in the lithium ion secondary battery to be discharged to the exterior, thereby ensuring the safety of the battery.
However, the safety vent 110 of the above lithium ion secondary battery has a thickness of about 20 μm, which is significantly smaller than the thickness (about 0.8 mm) of the cap plate 100, so the safety vent 110 may fracture even if external pressure is accidentally applied thereto, for example, when the lithium ion secondary battery is dropped.
Such a breakage of the safety vent 110 caused by external pressure may lead to leakage of the electrolyte contained in the lithium ion secondary battery, thereby degrading the safety of the lithium ion secondary battery.